1. Field of the Invention
This invention generally relates to a wheel driving device. More particularly, the present invention relates to a bicycle wheel driving device configured to drive a wheel of a bicycle using an electric motor.
2. Background Information
Bicycling is becoming an increasingly more popular form of recreation as well as a means of transportation. Moreover, bicycling has become a very popular competitive sport for both amateurs and professionals. Whether the bicycle is used for recreation, transportation or competition, the bicycle industry is constantly improving the various components of the bicycle. In recent years, bicycles equipped with a motor or the like are becoming increasingly popular to supplement the power imparted by pedaling (motor-assisted bicycles). Particularly in China, there is a marked increase in the popularity of electric powered bicycles that have an electric motor built into a hub of a wheel. These electric motors are configured either to assist with the rotation of the wheels or to drive the rotation of the wheels altogether such that the bicycle moves forward even if the rider does not pedal.
One conventional bicycle that uses a hub equipped with an internal motor (hereinafter called “motor-equipped hub”) is disclosed in Japanese Laid-Open Patent Publication No. 2004-25941. In this publication, a conventional motor-equipped hub comprises a hub axle and a casing to protect the hub axle. The casing comprises a stationary casing that is fixed to the hub axle and a hub shell (rotary drum) that covers the hub axle. Brake shoes of an internal expanding drum brake device are provided on the stationary casing. A brake drum for the internal expanding drum brake device is fixed to a cylindrical part formed so as to protrude from a bearing supported portion of the hub shell.
Such electric power bicycles are heavier because they require an electric motor and a battery serving as a power source. They can also travel at higher speeds than regular (non-motorized) bicycles because they are driven or assisted by the electric motor. Even though the inertia forces of electric powered bicycles are higher than those of normal (non-electric powered) bicycles, the electric powered bicycles are equipped with the same general-purpose brakes as regular bicycles, e.g., internal expanding drum brakes, band brakes, cantilever brakes, etc.
When a motor-equipped hub is installed on a bicycle, the mass of the bicycle increases and the bicycle is enabled to travel at higher speeds. Consequently, a brake device capable of delivering a larger braking force is preferable. However, conventional motor-equipped hubs are provided with internal expanding drum brake devices, which have rocking (pivoting) brake shoes of which only a portion contacts the-brake drum when the brake device is operated. As a result, when the brake device is operated such that strong braking forces are developed, large amounts of heat are generated and the coefficient of friction declines, making it difficult to obtain a sufficient braking force in a stable manner.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved wheel driving device. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.